The present invention relates generally to diagnostic imaging equipment for the human eye and in particular to an optical coherence tomography machine using thermal stimulation.
Optical coherence tomography (OCT) is widely used to study retinal diseases, for example, including macular degeneration (AMD), diabetic retinopathy, and glaucoma. In OCT studies of the eye, light is projected into retinal tissue where it reflects off of boundaries between tissue layers in the retina having different indices of refraction. The length of time required for the reflected light to return can be used to produce a depth-image of tissue layers analogous to that obtained in ultrasonic imaging.
The high speeds of light propagation (compared, for example, to propagation speed of ultrasound) is accommodated by deducing light transit time indirectly by looking at interference between the outgoing light and returning light using an interferometer and subsequent processing.
Ideally, the different tissue layers in the retina could be “highlighted” by using a tissue-selective contrast material preferentially retained by the tissue of one layer and introduced into the retina or by using pigments naturally present in certain retinal layers. Unfortunately, conventional contrast materials suitable for use in the eye do not significantly alter the index of refraction of the tissue and are thus are largely ineffective in emphasizing particular tissue in OCT imaging, the latter of which relies on phase differences generated by reflections at boundaries between tissues having different indices of refraction rather than on absorption of the tissue or contrast material.